Anti-polysaccharide (PS) antibodies (Ab) are critical to host defense against encapsulated bacteria. The function of anti-PS Ab includes complement fixation and opsonization of bacteria for killing by phagocytes, and modulation of cytokine producing cells via binding to Fcgamma receptors (FcgammaR). IgG anti-PS Ab response has delayed ontogeny and isotype restriction (IgG3 in mice IgG2 in man), and conjugation of PS to proteins induces class switching to IgG1. Determining the effect of isotype restriction and IgG subclass on function of anti-PS Ab is crucial to better understand immunity to PS-encapsulated bacteria, the pathogenesis of human IgG subclass deficiency and for improved serological correlates of immunity with PS and PS conjugate vaccines. In this proposal, we will continue our studies of the role of IgG subclass in anti-PS Ab effector function, determine the role of constant region genes and the dominant IgG subclass in immunity to encapsulated bacteria and in class switching that occurs when PS are conjugated to proteins, and investigate the effect of lgG subclass on FcyR mediated regulation of cytokine production. We will utilize V region-identical human monoclonal Abs of all four IgG subclasses against P. aeruginosa LPS O-side chain and S. pneumoniae (Pn) capsular PS made in a new transgenic mouse reconstituted with human Ig genes, to determine the mechanism of functional differences in Ab protective efficacy. Next, to determine the in vivo relevance of the dominant IgG subclass made to PS, we will use the new BALB/c IgG3 knockout mouse to determine the importance of anti-PS Ab subclass in the host response to PS, PS-protein conjugates and to infection with encapsulated bacteria. We have found that the absence of IgG3 renders these animals more susceptible to fatal infection with Pn but that induction of anti-PS IgG1 can correct this defect. Finally, we will investigate the FcR mediated regulatory role of IgG subclass in cytokine production by macrophages, and we hypothesize that there are differences in the ability of IgG subclass to modulate cytokine production based on differential binding to FcgammaR. These studies will allow more rational strategies of active and passive immunization against bacteria, improved understanding of IgG subclass deficiency, and new information about which IgG subclass has the best FcgammaR-mediated anti-inflammatory properties.